1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method for printing an image on a printing medium by ejecting ink from an ejection opening of a print head on the basis of print data.
2. Description of the Related Art
The level of requests submitted for the performances of printing apparatuses, such as printers, copiers and facsimile machines, has been remarkably increased in recent years, and now, entries for such requests may include not only rapid and full color printing, but also high-definition printing providing the quality equivalent to that associated with silver halide film printing. Today, an ink ejection type printing apparatus (an inkjet printing apparatus) can comply with such a request because technology permits the formation, on an inkjet print head, of an array of nozzles for the high frequency ejection of tiny ink droplets, and can thus provide superior printing speeds and printed image quality. Especially for a thermal inkjet printing apparatus, i.e., an apparatus that employs a print head in which heaters (electro-thermal converters) generate ink bubbles used to eject ink through nozzles, since nozzles can be arranged at a high density, a high resolution image can be provided.
Such a thermal inkjet printing system has the following two features.
First, in the thermal inkjet printing system, thermal energy is generated by supplying power to heaters that produce ink bubbles for ejecting ink droplets, and in the event, the growth of bubbles is greatly affected by the temperature of the ink in the immediate vicinity of the heaters. The process by which ink molecules, in a gaseous form, are released in the ink, and the process by which ink molecules, in a liquid form, are impelled to the bubble are performed at the interface between the bubble and the ink, and the temperature of the ink in the vicinity of the bubble greatly affects the performance of the second process. Therefore, when the temperature of the ink is high, since many ink molecules are released to the bubble, the bubble grows until comparatively large. On the other hand, when the temperature of the ink is low, since fewer molecules are released to the bubble, the size of the bubble produced is comparatively smaller. Thus, the size of a bubble affects the volume of ink pushed out by the bubble (hereinafter this volume of ink is called an “ejection volume”). Therefore, in the thermal inkjet printing apparatus, since the ejection volume of ink is greatly affected by the temperature of the ink near the heater, there is a tendency to increase the ejection volume when the temperature of the ink is high, and to reduce the ejection volume when the temperature is low.
In the thermal inkjet printing system, the ink temperature near the heater might become higher than before the start of printing as stated below.
Not all of the thermal energy generated by the heater contributes to the generation of bubble. The thermal energy remaining, after the energy required to generate bubble has been subtracted from the total generated, is stored as thermal energy in the surrounding ink or in the body of a print head member. As a result, the ink temperature near the heater is raised, and the stored thermal energy is released by heat transfer via a heater chip, where the heater is provided, or by the ejection of ink. However, since thermal energy is supplied by the heater during the printing operation, the temperature may continuously rise when the amount of the energy released is smaller than the amount of that supplied. On the other hand, during a non-printing operation, such as a printing medium conveying operation, in which thermal energy is not being supplied by the heater, the temperature near the heater could gradually fall until a thermodynamic equilibrium is established between the heater and its environment. Therefore, depending on the number of times individual heaters are driven, i.e., depending on the volume of the print data provided for individual nozzles, the temperature of some portions of the print head and nearby ink may be raised, while the temperature of the other portions and nearby ink may be reduced to around room temperature. Such high and low temperature portions of the print head could appear during the printing on a printing medium of a single page.
Because of the two above described features of the thermal inkjet system, when specific print data for one page are being printed on the printing medium, the temperature of the ink near the heaters may be raised in some portions and may be reduced in others, and different volumes of ink would be ejected from the nozzles in the high temperature portions and in the low temperature portions. Especially when the ejection volume is fluctuated while an image is printed on the printing medium based on print data, there is a possibility of changing the dimensions of dots formed by the ink landed on the printing medium. In this case, there is a possibility of giving rise to unevenness of the density distribution of images printed on a page to cause image deterioration.
To solve the problem of fluctuation in the ejection volume of ink due to the temperature of the print head, there is a well known method whereby the print head is maintained at a high temperature to control the fluctuation. For example, a printing apparatus disclosed in Japanese Patent Laid-Open No. 2006-334967 estimates heat exhausting effects by employing information (printing duty) about the volume of the ink required for printing and information about a temperature difference between the temperature of a print head and the temperature of the ink supplied to the print head. The change of the temperature of the print head is assumed based on the estimate, and heating power required to maintain the temperature of the print head is determined, so that the temperature of the print head is adjusted within a specific range.
However, when printing duty is high, i.e., when the ejection volume is large relative to a unit printing area, the temperature, affecting the ejection volume, of ink around the nozzle is reduced by discharging ink through the nozzle, that has a temperature lower than that of the print head. Therefore, when the temperature of the print head is maintained constant as in Japanese Patent Laid-Open NO. 2006-334967, the ink ejection volume and the ink ejection speed may fluctuate. Thus, the temperature of the print head must be set in accordance with the printing duty.
Further, in Japanese Patent Laid-Open No. 2006-334967, no description is given for a print head wherein nozzles in a plurality of sizes are formed for one ink liquid chamber. Assume that a nozzle having an ejection volume of 5-pl (pico liter) ink and a nozzle having an ejection volume of 2-pl ink are formed for one ink liquid chamber, and that the sizes of these two nozzles are different. In this case, when the ink ejection volumes of these nozzles are simply added together in accordance with the printing duty, the temperature of the print head cannot be accurately controlled. This is because the amounts of heat stored in 5 pl of ink and in 2 pl of ink are changed in accordance with the printing duty, respectively. When the printing duty is low, little heat exhausting effect can be expected when ejecting ink through the nozzles. Furthermore, more ink ejection energy is required for 5 pl of ink than for 2 pl, and a larger amount of heat is stored in the nozzle that ejects 5 pl of ink than in the nozzle that ejects 2 pl of ink. When the printing duty is increased, because greater heat exhausting effects can be obtained for nozzle that ejects 5 pl of ink, accordingly, heat stored in the nozzle can be greatly reduced, and in the end, may be less than the amount of heat stored in nozzle that ejects 2 pl of ink. Therefore, when the ink volumes ejected through the nozzles for 5 pl and 2 pl are simply added together, and the total ink volume is employed to select electric power for heating the print head from one table, temperature control for the print head is not appropriately performed.